Internal combustion reciprocating engines have been used for many years as electric power generation prime movers or directly for supplying industrial power needs. In these applications they are normally fueled by natural gas, diesel fuel or other conventional fuels. In some cases they are also fueled by combustible process gases produced as a byproduct from other manufacturing processes. In these cases the specific heating value of the process tailgas has typically been greater than 150 British thermal units (Btu) per standard cubic foot of gas (scf).
The tailgas produced as a byproduct of carbon black manufacture is typically composed of the following:
Hydrogen 6.5%-12% Carbon Monoxide 6.5%-12% Nitrogen 30%-62% Water 30%-45% Acetylene and less than 1% Higher Hydrocarbons
While the acetylene and higher hydrocarbons make up less than 1% of the tailgas by volume, they account for nearly 10% of the heating value of the tailgas.
The relative amounts of hydrogen and carbon monoxide can vary with the feedstock used and the carbon black grade produced but the heating value of the tailgas produced will usually be in the range of 50-65 Btu per scf as it leaves the process.
Historically, spark ignited reciprocating engines utilizing gaseous fuels have operated with air/fuel ratios slightly leaner than stoichiometric ratios and have been designed with natural gas as the base fuel. Natural gas typically has a net heating value of approximately 900 Btu per scf. When a fuel with a significantly lower specific heating value is used, the engine is not be able to achieve its rated capacity.
Recently, industrial reciprocating engines have been designed to operate with much leaner fuel mixtures in order to reduce emissions of nitrous oxides. The premise is that by increasing the uncombusted mass flowing through the cylinder, the peak combustion temperature is suppressed which in turn suppresses the formation of the nitrous oxides.
It has been discovered that the tailgas from a carbon black manufacturing process can itself be processed and used as a fuel in an industrial reciprocating engine.